In what political observers called a shocking display of anger from a former President of the United States, Bill Clinton today keyed the car of Illinois Senator Barack Obama.

Mr. Clinton’s attacks on Senator Obama have become more scathing in recent days, but few Democratic insiders expected his rhetorical attacks to turn into outright vandalism.

That is precisely what happened, however, in the parking lot of a Dunkin’ Donuts in Nashua, New Hampshire, where Mr. Obama and his aides had stopped for an early morning campaign appearance.

Spotting the Illinois senator’s car in the lot, a wild-eyed Mr. Clinton pulled out his key ring and “started twirling it on his finger like a six-shooter,” according to one eyewitness.

Saying he was “damned sick and tired” of everything going Mr. Obama’s way, the former President dragged his keys across the length of the senator’s car, creating a deep gash in the paint job that experts said would cost hundreds of dollars to repair.

As news of Mr. Clinton’s attack on Mr. Obama’s automobile spread like wildfire across New Hampshire, political insiders branded the former president’s move as a tactical mistake that could turn off Democratic voters.

“Keying another candidate’s car is really beneath the dignity of a former President of the United States,” said Carol M. Foyler, a longtime media advisor to Democratic candidates. “That’s the kind of thing you want surrogates to do.”

For his part, Mr. Clinton was unrepentant, telling reporters “you ain’t seen nothing yet.”

“Where does he live?” Mr. Clinton shouted at the press corps. “I’ma go TP that bastard’s house.”